A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by an electrical overload or short circuit. A circuit breaker automatically opens its contacts if an overcurrent condition is sensed. To do this, a circuit breaker comprises a trip unit, which determines when the contacts are to open.
Some circuit breakers include trip units including a thermomagnetic tripping mechanism. Such breakers are well known in commercial and industrial applications. These breakers include multi-metallic strips (e.g., a strips of two or more metals with different thermal expansion rates bonded together) for triggering a thermal trip resulting from overload currents and a magnetic element for instantaneous trip resulting from short-circuit current surges. Some breakers use bimetallic, while others use tri-metallic strips that are fixed on one end (these are collectively referred to as “bimetal levers” herein). In other words, the thermomagnetic tripping mechanisms include elements designed to both sense the heat resulting from an overload condition and the high current resulting from a short circuit. In addition, some circuit breakers incorporate a “push to trip” button.
It is possible that a temperature profile for a thermomagnetic trip mechanism of a standard circuit breaker does not meet the requirements for different circuit breakers under different operating conditions. For example, increasing the temperature inside the thermomagnetic trip mechanism of a circuit breaker can generate higher temperatures on the lugs and current path that do not comply with temperature range ratings (e.g., temperatures out of the acceptable range) of the circuit breaker. While using an arrangement with a low temperature profile can reduce the heat applied to the current path of the circuit breaker, low temperatures in the bimetal lever of a thermomagnetic trip unit can result in an inadequate amount of bimetal deflection and an insufficient amount of pushing power to actuate the trip bar, latch, or latching mechanism. Although a limited amount of deflection of the bimetal lever can be compensated for when using a low temperature profile by using a calibration mechanism (e.g., an adjustment screw) to locate the end of the bimetal lever closer to the trip bar of the breaker, the deflection strength may still not be sufficient for the bimetal lever to produce enough force to rotate the trip bar and release the energy storage spring of the thermomagnetic tripping mechanism.
FIGS. 1 and 2 depict isometric and side views respectively of a circuit breaker with a conventional thermomagnetic tripping mechanism 100. The thermomagnetic tripping mechanism 100 includes an input plate 102, an output plate 104, a heater 106 and a bimetal lever 108 which receives indirect heating through the heater 106. The bimetal lever 108 can be attached to the heater 106 and/or a heater support plate 114. A trip bar 110 of the thermomagnetic tripping mechanism 100 rotates as soon as it is moved by the bimetal lever 108. Rotation of the trip bar 110 releases the energy storage spring 112 of the thermomagnetic tripping mechanism 100 to open the contacts of the circuit breaker. A thermal calibration screw 116 can be used to calibrate the thermomagnetic tripping mechanism 100 to increase or decrease the amount of deflection (and time) the bimetal lever 108 undergoes to contact the trip bar 110. The bimetal lever 108 is shown in its normal (non-deflected) position. If no current is flowing through the current path of the thermomagnetic tripping mechanism 100, the bimetal lever 108 is in a straight “normal” position.
If sufficient overcurrent flows through the circuit breaker's current path, heat build-up causes the bimetal lever 108 of the thermomagnetic tripping mechanism 100 to deflect. As the bimetal lever 108 is heated, it bends from its high thermal expansion side toward its low thermal expansion side. After bending a predetermined distance, the bimetal lever 108 contacts and pushes on the trip bar 110 activating the energy storage spring 112 and thus the trip mechanism of the circuit breaker. Typically, if approximately 20% current over the nominal current rating of the breaker flows through the current path, the bimetal lever 108 generates pushing force based on the heat generated which rotates the trip bar 110 and releases the energy storage spring 112. Therefore, to insure reliable operation of thermomagnetic circuit breakers, systems, apparatus, and methods for improved tripping of circuit breakers are desirable.